POLI di MI tecnicolanotecnicolanotecnicolano INTEGRATED ACTIVE AND PASSIVE LOAD CONTROL IN WIND...

INTEGRATED ACTIVE AND PASSIVE LOAD CONTROL IN WIND TURBINES

C.L. Bottasso, F. Campagnolo, A. Croce, C. TibaldiPolitecnico di Milano, Italy

SAICA 20117-8 November 2011, Barcelona, Spain

POLITECNICO di MILANO POLI-Wind Research Lab

Need for Load MitigationTrends in wind energy:

Increasing wind turbine size ▶Off-shore wind ▼

To decrease cost of energy:• Reduce extreme loads• Reduce fatigue damage• Limit actuator duty cycle• Ensure high

reliability/availability

Presentation Outline

Load mitigatio

Active control

Passive control

Blade design optimization

Passive blade design

Integrated active/passive

control Load mitigatio

Active control

Passive control

control

Active Load Mitigation: Pitch Control

Individual blade Pitch Control (IPC)

Inner loop (collective pitch): regulation to set point and alleviation of gust loads

Outer loops (individual pitch): reduction of

- Deterministic (periodic) loads due to blade weight and non-uniform inflow

- Non-deterministic loads, caused by fast temporal and small spatial turbulent wind fluctuations

▼ Uniform wind ▼ Turbulent wind

Active Load Mitigation: PredictiveLiDAR-Enabled Pitch Control

LiDAR: generic model, captures realistically wind filtering due to volumetric averaging

Receding Horizon Control: model predictive formulation with wind scheduled linear model, real-time implementation based on CVXGEN

Non-Homogeneous LQR Control: approximation of RHC, extremely low computational cost

LiDAR prediction span

Reduced peak values

Reduced peak to peak oscillations

Flow control devices:• TE flaps• Microtabs• Vortex generators• Active jets

(plasma, synthetic)

• Morphing airfoils• …

Active Load Mitigation: Distributed Control

(Chow and van Dam 2007)

(Credits: Smart Blade GmbH)

(Credits: Risoe DTU)

Pitch control:• Limited temporal bandwidth (max pitch rate

≈ 7-9 deg/sec)• Limited spatial bandwidth (pitching the whole

blade is ineffective for spatially small wind fluctuations)

Distributed control:• Alleviate temporal and spatial bandwidth

issues• Complexity/availability/maintenance

Sensor-enabled control solutions:• Complexity/availability/maintenance

Active Load Mitigation: Limits and Issues

Off-shore: need to prove reliability, availability, low maintenance in hostile environments

Load mitigatio

Active control

Passive control

Active control

Passive control

control

Passive control: loaded structure deforms so as to reduce loadTwo main solutions:

Potential advantages: no actuators, no moving parts, no sensors

Other passive control technologies (not discussed here):- Tuned masses (e.g. on off-shore wind turbines to damp nacelle-tower

motions)- Passive flaps/tabs- …

Passive Load Mitigation

Angle fibers in skin and/or spar caps

- Bend-twist coupling (BTC): exploit anisotropy of composite materials

- Swept (scimitar) blades

Present study:• Design BTC blades (all satisfying identical design

requirements: max tip deflection, flap freq., stress/strain, fatigue, buckling)

• Consider trade-offs (load reduction/weight increase/complexity)

• Identify optimal BTC blade configuration• Integrate passive BTC and active IPC• Exploit synergies between passive and active load control

Baseline uncoupled blade: 45m Class IIIA 2MW HAWT

Objectives

Load mitigatio

Active control

Passive control

Active control

Passive control

control

Optimizer • Local/global solvers (SQP, GA)• Functional approximators

Optimization-Based Multi-Level Blade Design

1. Aerodynamic Optimization

2. Structural Optimization

+ Controls

3. Combined Aero-Structural Optimization

Cp-Lambda aero-servo-elastic multibody simulator

2D ANBA cross sectional analyzer

3D FEM models

Parameters

Cost function & constraints

Cost: AEPAerodynamic parameters: chord, twist, airfoils

Cost: Blade weight (or cost model if available)Structural parameters: thickness of shell and spar caps, width and location of shear webs

Cost: AEP/weigh (or cost model if available)Macro parameters: rotor radius, max chord, tapering, …

Controls:model-based (self-adjusting to changing design)

“Fin

level: 3

“Coars

level: 2

- Definition of complete HAWT Cp-Lambda multibody model- DLCs simulation- Campbell diagram

DLC post-processing: load envelope, DELs, Markov, max tip deflection

- Definition of geometrically exact beam model- Span-wise interpolation

- ANBA 2D FEM sectional analysis- Computation of 6x6 stiffness matrices

Definition of sectional design parameters

Constraints:- Maximum tip deflection- 2D FEM ANBA analysis of maximum stresses/strains- 2D FEM ANBA fatigue analysis

- Compute cost (mass)

Automatic 3D CAD model generation by lofting of sectional geometry

Automatic 3D FEM meshing(shells and/or solid elements)

Update of blade mass (cost)

Analyses:- Max tip deflection- Max stress/strain- Fatigue- Buckling

Verification of design constraints

SQP optimizer

min cost s.t. constraints

Constraint/model update heuristic (to repair constraint violations)

When SQP converged

2MW 45m Wind Turbine Blade Currently undergoing certification at TÜV SÜD

CNC machined model of aluminum alloy for visual inspection of blade shape

Design developed in partnership with Gurit (UK)

Load mitigatio

Active control

Passive control

Active control

Passive control

control

Fully Coupled Blades

1. Identify optimal section-wise fiber rotationConsider 6 candidate configurations

BTC coupling parameter:

Spar-cap angle

Skin angle

Fully Coupled Blades: Effects on Weight

Spar-caps: steep increase

Skin: milder increase

Spar-cap/skin synergy

Stiffness driven design (flap freq. and max tip deflection constraints):

Need to restore stiffness by increasing spar/skin thickness

Fully Coupled Blades: Load Reduction Spar-cap/skin synergy:

good load reduction with small mass increase

Fully Coupled Blades: Mechanism of Load Reduction

Fully Coupled Blades: Effects on Duty Cycle

◀ Less pitching from active control because blade passively self-unloads

Much reduced life-time ADC ▶

Partially Coupled Blades

2. Identify optimal span-wise fiber rotation: 5 candidate configurations

Reduce fatigue in max chord region

Avoid thickness increase to satisfy

stiffness-driven constraints

Partially Coupled Blades: Effects on Mass

Too little coupling

Fully coupled blade

Partially Coupled Blades:Effects on Loads F30: load reduction

close to fully coupled case

Partially Coupled Blades:Effects on Duty Cycle

Best compromise: similar load and ADC reduction as fully coupled blade, decreased mass

Load mitigatio

Active control

Passive control

Active control

Passive control

control

Integrated Passive and Active Load Alleviation

Individual blade pitch controller (Bossaniy 2003): • Coleman transform blade root loads• PID control for transformed d-q loads• Back-Coleman-transform to get pitch inputs Baseline

controller:MIMO LQR

Integrated Passive and Active Load Alleviation

Two IPC gain settings:1. Mild: some load reduction, limited ADC increase2. Aggressive: more load reduction, more ADC increase

Five blade/controller combinations:• BTC: best coupled blade + collective LQR• IPC1: uncoupled blade + mild IPC• BTC+IPC1: best coupled blade + mild IPC• IPC2: uncoupled blade + aggressive IPC• BTC+IPC2: best coupled blade + aggressive IPC

Integrated Passive/Active Control:Effects on Loads

▲ Synergistic effects of combined passive and

active control ▶

Integrated Passive/Active Control:Effects on Duty Cycle

Not significant: ADC very small

Same ADC as baseline (but great load

reduction!)

• Optimization-based blade design tools: enable automated design of blades and satisfaction of all desired design requirements

• BTC passive load control: - Skin fiber rotation helps limiting spar-cap fiber angle- Partial span-wise coupling limits fatigue and stiffness

effectsReduction for all quality metrics: loads, ADC, weight

• Combined BTC/IPC passive/active control:- Synergistic effects on load reduction- BTC helps limiting ADC increase due to IPC (e.g., could

have same ADC as baseline blade with collective pitch control)

Outlook:• Manufacturing implications of BTC and partially coupled

blades• Passive distributed control and integration with blade

design and active IPC control

Conclusions

POLI di MI tecnicolanotecnicolanotecnicolano INTEGRATED ACTIVE AND PASSIVE LOAD CONTROL IN WIND...

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